John thomson



l. THOMSON.

ZIGZAG CARBON ELECTRIC RESESTER.

APPLICATION FILED 0'01. 1. 1918.

Fig L' f4 INVENTOR By 676' ATTORNEYS Tm; COLUMBIA PLANOURANI co., WASHXNHTON, u. c.

JOHN THOMSON, OF NEW YORK, N. Y.

ZIGZAG- CARBON ELECTRIC RESISTER.

Specification of Letters Patent.

Patented Oct. *7, 1919.

Application filed October 7, 1918. Serial No. 257,215.

T 0 all whom it may concern:

Be it known that I, JoHN THOMSON, a citizen of the United States, and a resident of the borough of Manhattan, city of New York, county and State of New York, have invented certain new and useful Improvements in Zigzag Carbon Electric Resisters, of which the following is a specification.

This is an invention in zig-zag, carbon, electric resisters and casings therefor, to be used in furnaces, and the objects of the invention are to prevent the oxidization of the resister and to provide means for its support when it is desirable to suspend the resister in the heating chamber of a furnace.

In the drawings,

Figure l is a vertical, longitudinal center section of a furnace in which an incased resister is suspended above a tank containing, for example, a bath of molten metal; the casing being shown in section on the lefthand side of the center line, A, and in sideelevation on the right hand side;

Fig. 2 is a transverse section through the resister and its casing upon a scale considerably larger than that of Fig. 1, and

Fig. 3 is a view similar to that of Fig. 2, but denoting certain modifications in the detail to be hereinafter pointed out.

Resistors of the material and type herein designated when unprotected cannot be used, except for short intervals of time, in furnaces whose heating chambers are constantly or intermittently exposed to atmosphere, because the oxygen combines with and destroys the carbon. Moreover, when the carbon element is slotted, to form an elongated zig-zag circuit, its stability is diminished; and, when highly heated, if sus pended by support at itsends, there is liability of sagging along its central portion.

The elemental features of the drawings are as follows: B is the furnace brick-work and C the roof forming a heating chamber D, containing a tank E in which is a bath of molten metal G, to be thermally acted upon. F is a longitudinally extending resister within an encompassing casing H, having an inclosing cover I. The ends, a, b, of the casing are supported in the furnace brick-work. The terminals, J, K, are attached to or are formed as a part of the resister; they lead to the outside and are symbolically denoted as connected to a power-circuit, as L. The vertically and transversely extending slits, 0, in the resister are staggered or alternately arranged and produce the sinuous or zig-zag path along which the current flows. The preferred cross-sectional form of the resister is trapezoidal, whereby its narrower surface or zone possesses the greater current-density with a correspondingly higher temperature and heat emissivity.

The preferred materials which have been ascertained to be uniquely adapted for constructing the resister-casing are alundum, that is fused aluminum oxid, or silicon carbid in its crystalline form, either of which, when combined with a binder, such as highgrade fire-clay, can be accurately molded; and articles made therefrom, when heattreated or cured, as in a kiln, acquire considerable transverse strength and density. These materials are sufficiently refractory to serve in the majority of cases; are good conductors of heat; but are relatively poor conductors of electricity; are non-reactive with carbon and are immune, or practically so, to oxidization.

Tile inner lower surface, d, of the casing and its sloped sides, 6, h, are preferably formed to the exact dimensions and contour of the resister, whereby the contiguous surfaces of the resister on the one hand and the casing on the other fit or come into physical contact, and the effect of the fit or contact may be augmented to the utmost extent of intimacy by interposing a thin coating or layer of finely pulverized alundum or carborundum powder. In this wise, the casing virtually becomes an integral part of the resister and the coefficient of transfer of heat, from the resister itself and thence through the wall of the casing, is about as high as is possible to attain. portion of the casing is then completed or inclosed by the cover-plate, as I, having beveled edges m, forming longitudinal recesses V shape in cross section filled with cement n. The casing is preferably formed so that when inclosed, as by the plate, there will be a clear space, as 1", above the resister, in which any evolved gases are trapped; or inert gases may be formed, as by inserting a mixture of ZnO+C, thereby preventing any infiltration of oxygen or, if present, combining therewith to form Zn+CO.

The upper This dead, gas-filled space also serves the beneficial function of obstructing the flow of heat upwardly.

The open casing and its inclosing cover, as depicted in Fig. 2, is regarded as the more advantageous structure when the alternate ends of the resister are connected to terminals, as in Fig. 1, in that both the cover-plate and the resister may thus be conveniently removed and replaced without disturbing the casing itself.

When, however, the resister is formed with zig-zagged limbs, as s, t, Fig. 3, produced by a longitudinal central slot, to (thereby producing a prong-like structure which in plan. is the form of a V with the terminals at the ends of the V, the circuit being in series and both terminals neces sarily contiguously located) the casing may then be molded after the manner of a closed mufl'le as illustrated in Fig. 3, in which case the resister can be conveniently slid-in or drawn out from. one of its ends.

Yet, either style of resister may be used in either of the casings illustrated. It is to be here observed that the cross-sectional form of the casings is well adapted to resist deformation, preventing the resister from sagging and maintaining close contact with its principal heat-transmitting sur" faces.

The wall of the casing need not have a thickness exceeding, say, three-eighths to five-eighths of an inch; consequently the lag in heat-fl0w is, in a practical sense, a negligible quantity.

The preferred material from which the resister is formed is graphitized carbon, and while its co-efficients of expansion and con traction are somewhat less than that of either alundum or recrystallized carborun dum, there is no liability of imparting destructive stresses from one member to another as the graphite is in itself a lubricant of high eiiiciency.

It might be assumed, upon first thought, that by placing the slitted zig-zags of the resister in contact with a casing, whatever its electrical non-conductivity, would result in a considerable diminution of its resistance, that is by short-circuiting and shuntage of current into the casing itself; but such is in *fact negligible, the reason thereof being that the E. M. F. between any contiguous pair of zig-zags is insufficient to produce a deleterious effect.

Some of the important advantages of this composite structure are as follows: Its first cost is nominal; its up-keep expense, during long periods of operation, is m'l and the relatively delicate, but highly efficient, zig-zag type of resister may thus be employed in services where, in the electrical sense, foolproof elements are essential to commercial success.

What I claim is:

1. A zig-zag, carbon, electric resister contained within a casing formed of material non-reactive with carbon and oxygen.

2. A zig-zag, carbon, electric resister contained within a casing formed of crystalline silicon carbid.

3. A zig-zag, carbon, electric resister whose cross-sectional contour is such as to produce a greater current-density along one or more zones of its circuit contained within a casing formed of material non-reactive with carbon and oxygen.

4:. A zig-zag, carbon, electric resister whose cross-sectional contour is trapezoidal, contained within a casing formed of material non-reactive with carbon and oxygen, the beveled sides and lower face of the re sister being in physical contact with corre sponding surfaces of the casin 5. A zig-zag, carbon, electric resister contained within a casing non-reactive with carbon and oxygen, the sides and bottoms of the resister and easing being in physical contact but having a free space between the upper face of the resister and the upper inclosing wall of the casing.

This specification signed and witnessed this 21st day of August, 1918.

JOHN THOMSON.

Signed in the presence of- RALPH M. THOMSON,

H. O. WEED.

Copies of this patent may be obtained for five cents each, by addressing the Commissioner of latents, Washington, I). G. 

